The present invention relates to an improved bituminous paving material and a method for the preparation thereof. More particularly, the invention relates to a bituminous paving material containing aggregates and having greatly improved resistance against plastic flow which may lead to the depression or raveling of the paved road by ruts in the road surface.
As is well known, most of the modern roads are paved with a bituminous or asphaltic paving material and can withstand fairly satisfactorily the severest traffic conditions such as in the high-speed motorways as a result of the remarkable progress in the technology of road construction while also ensuring traffic safety. Such progress in road technology has resulted from integration of the technical aspects of structure design, formulation, materials and construction techniques.
Despite the great efforts of road engineers, however, no asphaltic pavement can meet all of the requirements of conditions of climate, road bed, traffic frequency and the like which may differ very widely from place to place. In particular, the most difficult problems to be solved are the raveling of the pavement by ruts in hot regions and wearing and crack formation of the pavement in cold regions, for which new proposals are being made one by one, directed to improvements in the paving materials but which do not provide a complete solution to the above problems.
A bituminous material is usually composed of aggregates in a wide range of sizes from coarse aggregates and fine aggregates to a stone powder or so-called filler including smashed rock, gravel, sand, crushed sand and the like material and a bituminous material such as natural asphalt, petroleum asphalt and the like as the matrix for the aggregates.
The amount of the aggregate material in the overall paving material is usually 90% by weight or more, or about 75% by volume or more. Road surfaces paved with these asphaltic paving materials cannot be free from the problem of surface deterioration such as sliding, raveling by ruts, undulation and crack formation as a result of frequent heavy vehicle traffic, dynamic loading and sudden braking, changing road bed conditions, temperature changes and the like, and wear from studded tires. In particular, the most serious drawbacks of the bituminous-paved road are depression by rutting in hot regions and wearing and crack formation in the road surface in cold regions.
With an object to solve the above mentioned problems, various attempts have been made to improve bituminous paving materials, as a first approach, by means of an appropriate selection of the aggregate materials and use of an asphalt-modifier agent. In particular, the aggregate material greatly influences the performance of the paving material since the aggregate material occupies about 75% or more of the overall volume of the paving material. In this regard, aggregate materials should be subject to most careful selection in respect of the particle size distribution, durability, wearing resistance, slipperiness, and content of mud and organic foreign materials, shapes of the particles, adhesion to asphalt and other characteristics. A guideline therefor is given in "Asphalt Paving Manual" published by the Japanese Road Association.
A second approach for the improvement of the asphaltic paving materials is concerned with the modification of the bituminous material as the binder of the aggregate and which occupies about 25% by volume of the paving material. For example, several improved bituminous materials suitable for pavement have been proposed including so-called semi-blown asphalts having increased viscosity at 60.degree. C., rubber-containing asphalts mainly directed to the improvement of the toughness and tenacity, thermoplastic resin-containing asphalts mainly directed to the improvement of the flow resistance at elevated temperatures and the like as well as asphaltic materials used in combination with an epoxy or urethane resin as a specific organic binder.
In connection with the above described first approach for the improvement of the asphaltic paving materials, the proper selection of the aggregate material in respect of the particle size distribution is a matter to be determined according to the required density or use of the pavement as a necessary condition rather as an improvement. However, aggregate quality is frequently uneconomical unless the aggregate material is obtained near the site of the road construction and aggregate materials of low quality require expensive processing for improvement.
In connection with the second approach above described, most of the proposed methods for the improvement of the asphaltic material are directed in principle to increase of viscosity. For example, the semi-blown asphalt is prepared by blowing air into a molten straight asphalt for a long time so as to improve the heat sensitivity with an increased viscosity at 60.degree. C. with an object to improve the flow resistance as a result of the increased viscosity. The rubber blended with the asphalt in the rubber-containing asphalt is partly dissolved in the asphalt but mostly dispersed in the matrix of the asphalt in the form of swollen rubber particles contributing to the improvement of the toughness and tenacity of the asphalt by virtue of the increased resistance against flowing. The function of the thermoplastic resin blended in the thermoplastic resin-containing asphalt is much the same as the above mentioned rubber in the rubber-containing asphalts as is taught in the "Asphalt Paving Manual".
On the other hand, some epoxy and urethane resins have chemical reactivity with asphalt as a specific additive to form a very strong asphalt concrete and many disclosures are given in the prior art. A problem in the additive resins of this type is the relatively short time available for working with the ready-mixed asphalt mixture since the reaction of the resin and the molten asphalt proceeds to reduce the workability of the mix.
Thus, all of the prior art methods for the improvement of asphalt by the addition of certain additives are directed to the hardening of the asphalt itself by the dispersion of a heterophase or by chemical reaction.